This invention relates to an interface for a liquid chromatograph and a mass spectrometer (LC/MS), and particularly to an improvement of an interface using ultrasonic vibrator for atomizing an effluent from the liquid chromatograph (LC).
An atmospheric pressure ionization mass spectrometer (APIMS) is a mass spectrometer (MS) capable of mass analyzing trace amounts of samples by efficiently ionizing the sample at the atmospheric pressure by a corona discharge or by .beta.-rays emitted from .sup.63 Ni. A liquid chromatograph (LC) is device capable of separating and detecting nonvolatile or thermally labile compounds and has been so far widely utilized. However, it has been tried to combine a liquid chromatograph (LC) with a mass spectrometer (MS), because no highly sensitive detector for liquid-chromatograph (LC) is available and no information is obtained as to molecular structure, such as the mass of sample, etc. [Anal. Chem. 51 682A (1979)].
Atmospheric pressure ionization mass spectrometer (APIMS) is highly sensitive and can perform ionization at the atmospheric pressure, and thus is very convenient for the desired combination with the liquid chromatograph (LC). Horning et al proposed a liquid chromatograph/atmospheric pressure ionization mass spectrometer (LC/APIMS). [Anal. Chem. 47 2369 (1975); J. Chromatogr. 99 13 (1974)]. Since then, the present inventors also proposed an interface for LC/MS, where an effluent from the liquid chromatograph (LC) is atomized by an ultrasonic vibrator and mass-analyzed in the atmospheric pressure ionization mass spectrometer (APIMS). For example, such an interface for LC/MS is disclosed in Japanese Laid-open Utility Model Publication No. 167467 (1981). FIG. 1 shows a basic structure of the interface for LC/MS, where an effluent from liquid chromatograph (LC) column 1 in a liquid chromatograph 13 is continuously supplied to the surface of an ultrasonic vibrator 9 along a guide wire 10 for the LC effluent, and atomized on the surface of the ultrasonic vibrator 9, and the resulting mists are carried into the ion source 11 of an atmospheric pressure ionization mass spectrometer (APIMS) 15 comprising the ion source 11 and an analyzer 7 together with a carrier gas from a carrier gas inlet pipe 2.
The ion source 11 comprises a heater 5 for making the mists generated by the ultrasonic vibrator 9 finer, a corona discharge electrode 6 for ionizing the samples, and an outlet pipe 8 for eliminating an excess gas. The carrier gas and samples can be efficiently ionized at the atmospheric pressure. After the ionization, ions are carried into the analyzer 7 and mass-analyzed.
In the foregoing structure, the ultrasonic vibrator 9 is used, as such, for the atomizing, and thus the volume of the interface for LC/MS 14, that is, the dead volume indicated by dotted line 12 is increased by amount corresponding to the area of ultrasonic vibrator 9.
Generally, the diffusion rate is proportional to a difference in concentration between the two sides at the boundary surface, that is, a concentration gradient, and also to a cross-section of the boundary surface at the same time. To suppress the diffusion of a substance, the cross-section of the boundary surface must be thus reduced, and the time required for passing through the boundary surface must be shortened. That is, in the interface for LC/MS 14 the diffusion rate of a substance greatly depends on volume 12 of the interface. In other words, if the volume of the interface for LC/MS is large, as indicated by dotted line 12, samples successfully separated from one another in LC column 1 will be mixed together again due to the high diffusion rate. This has been a problem in the conventional interface.